Cooling of high heat flux electronic devices using ultra-thin multiport minichannel thermosyphon

Journal Pre-proofs Cooling of high heat flux electronic devices using ultra-thin multiport minichannel thermosyphon Stephen Manova, Lazarus Godson Asirvatham, Rajesh Nimmagadda, Jefferson Raja Bose, Somchai Wongwises PII: DOI: Reference:

S1359-4311(19)33745-7 https://doi.org/10.1016/j.applthermaleng.2019.114669 ATE 114669

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

30 May 2019 7 November 2019 10 November 2019

Please cite this article as: S. Manova, L. Godson Asirvatham, R. Nimmagadda, J. Raja Bose, S. Wongwises, Cooling of high heat flux electronic devices using ultra-thin multiport minichannel thermosyphon, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114669

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Cooling of high heat flux electronic devices using ultra-thin multiport minichannel thermosyphon [

Stephen

Manovaa,

Lazarus Godson Asirvathama,⁎, Rajesh Nimmagaddab

Jefferson Raja Bosea and Somchai Wongwisesc a

b

c

Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore 641 114, Tamil Nadu, India

Center for Advanced Energy Studies (CAES), Koneru Lakshmaiah University, Vaddeswaram, Guntur 522502, Andhra Pradesh, India

Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab (FUTURE), Faculty of Engineering, Department of Mechanical Engineering, King Mongkut’s University of Technology Thonburi, Bangmod, Bankok 10140, Thailand Abstract Studying multiport minichannels as thermosyphons are considered to be of great interest

compared to cylindrical and flat shaped ones with single ports. In this study, a novel multiport minichannel (MPMC) thermosyphon (hydraulic diameter of 1.18mm, length of 200mm with 10 internal ports with thickness 0.1mm each) with acetone as working fluid is experimentally investigated for cooling high heat flux electronic devices. Effect of heat load (10 to 50 W), filling ratio (40%, 50%, and 60%) and tilt angles (45o, 60o and 90o) on thermal resistance, convective heat transfer coefficients of evaporator and condenser, vapour velocity, flooding, sonic, boiling limits and efficiency are experimentally studied. Results showed 9.31% and 22.2% reduction in evaporator wall temperature and thermal resistance at optimum filling ratio (OFR) of 50%. Enhancement in evaporator heat transfer coefficient and thermal efficiency of 30.7% and 89.8% are respectively observed at OFR. Multiports present in minichannel acts as internal fins and increases the surface area and evaporation rate. Further multiports increases surface tension of condensate at right angles to the flow direction along with the effects of gravity enhancing the rate of condensation. Thus, the obtained experimental results will be useful in cooling of miniaturized high heat flux electronic devices. . Keywords: Multiport-minichannel, filling ratio, inclination angle, Acetone, flat thermosyphon

⁎Corresponding

author: [email protected], [email protected] (Lazarus Godson Asirvatham), Tel.: +91 422 2614430: fax: +91 422 2615615 Email addresses: [email protected] (Stephen Manova), [email protected] (Jefferson Raja Bose), [email protected] (Rajesh Nimmagadda), [email protected] (Somchai Wongwises) 1 Introduction The heat flux density in modern miniaturized electronic products exceed 100 W/cm2 at chip level and 30 W/cm2 at the heatsink base level and these figures are expected to be even greater in future electronic products [1]. Efficient thermal management to improve the reliability and enhance the cycle efficiency is necessary for these products. Maintaining the temperatures within the safe operating limit is a challenging task till date [2]. Conventional single-phase cooling methods involving air and liquid are inadequate for these devices operating at higher heat flux ranges. However, two-phase cooling provides a promising alternative. Two phase cooling technology with micro and minichannels have drawn a great attention as one of the effective methods in the field of electronic cooling [3]. Many researchers have studied boiling and condensation heat transfer with different working fluids in cylindrical and flat shaped thermosyphons that has only single ports. In recent years multiport minichannels are being considered to be of great interest in compact evaporator applications both in automotive air conditioning and heat pump systems [4]. Multiport minichannels have many advantages over the single channel such as more compact size, light weight, and more surface to volume ratio resulting in higher heat transfer. However, no work has been reported till date in the open literature to study the heat transfer performance characteristics of minichannel thermosyphon having multiports for cooling modern power electronic products. Thus, designs of multiport minichannels as thermosyphons is proposed to perform better compared to cylindrical and flat shaped thermosyphons that has only single ports. Some of the most productive works on heat transfer study of cylindrical and flat shaped thermosyphons particularly with single ports are summarized and described below. Jafari et al. [5] conducted experiments on two-phase closed thermosyphon (TPCT) with single port to determine its evaporation and condensation heat transfer coefficient. The tested TPCT has 500 mm length and 33 mm inner diameter. Experiments were carried out with different

filling ratio (FR) ranging from 8 to 100%. The film wise model consisting of liquid entrainment effects in condenser section showed good agreement with experimental results to predict condensation heat transfer. The authors suggested Chowdhury [6] and Shiraihi’s [7] correlations for predicting the heat transfer rate of thermosyphons at lower filling ratios. A comparative study on flat and cylindrical TPCT with water as working fluid was done by Amatachaya and Srimuang [8]. Results showed that the thermal performance of flat TPCT with single port is slightly higher by 5.6% when compared to cylindrical TPCT for 60% FR. Rittidech and Srimuang [9] conducted experiments to determine the effect of dimensionless parameters on the heat transfer performance of vertical flat thermosyphon. The flat thermosyphon was fabricated by pressing them in to mold with three different thicknesses such as 6.6, 4.6, 2.6 mm with water and ethanol as working fluids. Four FR’s from 20% to 80% of the total volume were tested. The effect of dimensionless parameters such as bond number, Prandtl number, Kutateladze number and Jacob number on heat flux were discussed. Moreover, the authors developed a new correlation based on these dimensionless parameters to predict the heat flux of a vertical flat thermosyphon.

Mehmet and Hikmet [10] experimentally studied the thermal performance of two-phase thermosyphon (TPT) solar collector with three different refrigerants. The refrigerants such as R134a, R407C and R410A were tested under various environmental conditions. Refrigerants were filled inside the thermosyphon having single port in which the condenser section was immersed in a water bath which was kept at 25 °C. From the result, it was noted that R410A has the maximum capability of absorbing the solar thermal energy as the water temperature raised from 35.5 to 64.4 °C. Moreover, the collection efficiency ranged between 50.86% and 57.13% for R410A at peak hours. Similar work on using refrigerants as working fluid was experimentally studied by Ma et al. [11]and the obtained heat transfer coefficient values have good agreement with Kaminaga and Hashimoto correlations [12]. Ong et al. [13] studied the thermal performance of single port thermosyphon filled with R410A for low heating applications. The heat load varied from 40W to 100W and inclination ranged between θ=30o and θ=90o. However, a contradictory result was observed by the authors stating that the FR and inclination angle had negligible effects on heat transfer performance. Alammar et al. [14] studied the thermal performance of thermosyphon by making the internal surface rough throughout the channel. Heat load was varied from 90 to 160W

and water was used as a working fluid with constant FR of 50%. Experiment was carried out at two pressure levels such as 3 kPa and 30 kPa. Results showed that, the thermosyphon with internal rough surface displayed a decrease in thermal resistance of 42% and 13% for the tested pressures of 3 kPa and 30 kPa. On the other hand, the heat transfer coefficient increased to a maximum of 68% for 30 kPa pressure. Shafi et al. [15] experimentally investigated the performance of novel magnetic variable conductance thermosyphon heat pipe (MVCTHP) and studied the effect of FR, evaporator length and the inclination angle. A steel ball was placed inside the evaporator and performance was observed by adjusting its position. The heat load varied from 30 to 120 W with three inclination angles 30o, 60o, 90o. Results concluded that the best performance was observed for 20% FR with 90o inclination and the lowest thermal resistance value was noted as 0.71°C/W for the minimum ball position. Naresh and Balaji [16] conducted experiments on the performance of internally finned thermosyphon. Three FR’s such as 20%, 50%, 80% with water and acetone as working were used for the experiment. From the results, it was noted that 50% FR yielded higher heat transfer performance. As fins were attached internally to the condenser section of thermosyphon, enhancement in condensation rate of about 17% was observed. Moreover, a reduction of 35.48% in thermal resistance was also observed for optimum filling ratio at higher heat inputs. Solomon et al. [17] experimentally studied the thermal performance of cylindrical anodized TPCT with single port. A comparative study with anodized and non-anodized one was done with heat load ranged from 50 to 250 W. From the experimental results, it was observed that the nucleation sites in anodized TPCT were 2 to 3 times higher than the non-anodized TPCT. Moreover, a reduction in thermal resistance ranged from 1.5% to 15.01% was observed for heat loads 50W to 250W. The authors stated that a decrease in thermal resistance was due to an increase in the number of nucleation sites. Similar work was done by Renjith et al. [18] to find the effect of anodization on single port flat thermosyphon with uniform coating on the inner surface. Comparative study between flat and cylindrical thermosyphons having single ports was done and results showed that flat thermosyphon performed well. Moreover, enhancement of 69% in the evaporator heat transfer coefficient was observed at 50 kW/m2. Kim et al. [19] studied the effect of sintered microporous coating at the evaporator section of TPCT. Micro-sized copper particles were coated on the inner surface of the evaporator with a coating thickness of 500µm. The FR from 25% to 100% and different inclination angles ranged from θ=90o to θ=5o were experimentally

tested. From the results, it was observed that the overall thermal resistance reduced by 51% for 35% FR and 30% for 75% FR. Moreover, the higher thermal performance was noted for 15 o and 30o inclination angles. Further, some works were also reported in the open literature [20]–[28] that discuss on enhancing the thermal performance of thermosyphon/heat pipes using nanofluids in addition to the use of water and refrigerants. The authors have carefully reviewed the information available in the open literature. Based on the survey, it is observed that there are some works reported in the literature, that deals with the heat transfer performance characteristics of conventional thermosyphons having cylindrical and flat geometries with only single ports. Further, there are many literatures available on minichannel and microchannel heat exchangers, even with multiports that deals with flow boiling heat transfer characteristics of different working fluids under constant wall heat flux and constant wall temperature boundary conditions. To the best of authors knowledge, no work has been reported in the open literature that investigate the heat transfer performance of multiport minichannel thermosyphon for the cooling of high heat flux miniaturized electronic products. Before employing any heat exchange system into practical use, a study on their heat transfer performance characteristics is necessary. Besides, a study on multiport minichannel thermosyphon for different heat load, filling ratios and inclination angle remains undone. As a consequence, the present experimental work aims to determine the heat transfer performance characteristics of ultra-thin multiport minichannel thermosyphon (hydraulic diameter of 1.18 mm) using Acetone as the working fluid for miniaturized power electronic products. The effect of heat load (10 W to 50 W), filling ratio (40%, 50%, and 60%) and tilt angles (45o, 60o and 90o) on thermal resistance, convective heat transfer coefficients of evaporator and condenser, vapour velocity, flooding, sonic, boiling limits and efficiency which have not been reported before in the open literature especially for the multiport minichannel thermosyphon are presented. 2 Experimentation 2.1 Experimental setup. The MPMC thermosyphon is comprised of three parts namely: evaporator, adiabatic and condenser sections respectively as shown in Figure 1. The complete details of MPMC thermosyphon are given in Table 1. All the parts of thermosyphon is made of Aluminium and the condenser is made of acrylic material. There are 10 multi-ports channels in the thermosyphon and

every channel is separated by individual separators. A flat type thermosyphon is chosen as it offers greater surface area and very less thermal resistance with the applied heat load. The evaporator wall has a uniform thickness of 0.5mm throughout its length which has the capability to withstand the vapor pressure created inside the thermosyphon. The total height of thermosyphon is fixed as 200 mm in which the length of the evaporator, adiabatic and condenser section are 75 mm, 25 mm and 100 mm respectively. The ends of the mini channel are closed with the end closures by TIG welding process and perfect sealing has been ensured. At the condenser section, a small drill has been made for fixing the filling tube through which the working fluid is filled in the thermosyphon with the required filling ratio. Before the filling process is carried out, the leak test was conducted in the thermosyphon using the helium leak detector to ensure that the thermosyphon has leak-free joints. After ensuring that the thermosyphon is leak-free, it is then attached to a vacuum pumping system, where the vacuum is created up to a pressure of 10-4 mbar. When the high vacuum is attained (pressure of 10-4 mbar), the working fluid is filled for the required filling ratio and finally, the filling tube of the thermosyphon is sealed using a crimping press. The working pressure of the thermosyphon differs for each heat load and it corresponds to the saturation temperature. For the heat load from 10W to 50W (510 W/m2 to 2551 W/m2) which is been tested in this experiment, the working pressure ranges between 0.45 bar to 1.6 bar. The experimental setup consists of the following components: MPMC thermosyphon with condenser attached, heating coil, dimmerstat, voltmeter, ammeter, chilling unit, thermocouples, datalogger, rotameter, pump, and a personal computer. Figure 2. represents the experimental setup of the MPMC thermosyphon with port dimensions. The heater coil consists of two layers of mica sheet and Nichrome wire which is surrounded evenly on the evaporator wall so that uniform heat along the surface can be supplied. The heat loss from the wall of the thermosyphon is shielded to a maximum extent using glass wool which prevents the heat leakage to surrounding. Input heat is varied using dimmerstat and the corresponding heat load is calculated using voltmeter and ammeter readings. The inlet and outlet tubes are connected to the hose collar that is fixed in the condenser. The cooling water is made to flow in the direction against the gravity by which enhanced heat transfer occurs. Along with the surface temperature, the inlet and outlet tubes of cooling water tubes were attached to the thermocouples. Twelve T-type thermocouples in which, ten thermocouples on the thermosyphon and remaining two to find out the temperature of the inlet and outlet temperature of the cooling water. The T-type thermocouples that are used in this

experiment have an accuracy of ±0.5°C. The position of thermocouple for each section of thermosyphon is discussed below. Four thermocouples named are fixed at the evaporator surface, two thermocouples are placed at the adiabatic wall whereas four thermocouples are placed on the wall of the condenser section by which the temperature of the fluid that is descending down to the evaporator. Moreover, the mass flow rate of the cooling water is set to 10 LPH. The inlet and outlet temperature of the cooling water is measured using the thermocouple (T6 and T7). Twelve thermocouples were connected with the datalogger by which the measured temperature was recorded in the personal computer for later analysis.

2.2 Experimental procedure. An experimental study on heat transfer performance of MPMC thermosyphon was carried out with Acetone as a working fluid. Tests were carried out for different FR and inclination angles. Initially, the optimization of FR is been carried out and then an optimized inclination angle has been found out. FR is calculated as the ratio of the amount of the working fluid present in the channel to the total volume of the multi-port mini channel thermosyphon. Three different FR of 40%, 50%, and 60% were conducted and the latter part of this work deals with the three inclination angles of 45o, 60o and 90o (vertical orientation). The experimental procedure is carried out as follows: The MPMC thermosyphon with heating coil winded is connected with the dimmerstat and all the twelve thermocouples are connected with the datalogger. The time interval is set to 10s for the datalogger to record all the temperatures. Cooling water from the chilling unit is made to pass through the tubes and then to the condenser. The temperature of the cooling water is set to 20 °C and the mass flow rate is adjusted by the rotameter and is set to 10LPH. Cooling water supply is continued for 20 minutes and this is to ensure that the thermosyphon's temperature reaches its steady state. Once the steadystate is achieved, the heat load of 10W is applied. This process continues until the evaporator wall temperature becomes steady and for each heat load constant period of 30 minutes is maintained. Then the heat load is increased to 50W with an interval of 10W and the average values are taken for further analysis. OFR is determined by finding the minimum thermal resistance value. The

second phase is carried out for the optimum FR with respect to three different inclination angles. The MPMC thermosyphon is tilted for the required angles and then experiments were carried out. A study on heat transfer is conducted for each inclination and then the thermosyphon is detached from the experimental setup by removing the thermocouples from the datalogger. Finally, the thermosyphon is cooled until it reaches the ambient temperature. 2.3 Data Reduction The resistance value of thermosyphon (Rth) is defined as the ratio temperature gradient between the evaporator and the condenser section to the applied heat load (Qin) and it is calculated as in Eq. (1). 𝑅th =

𝑇e,w − 𝑇c,w 𝑄in

(1)

Where Te,w and Tc,w represents the wall temperatures of evaporator and condenser section respectively and it can be calculated as follows. 𝑇e,w = 𝑇c,w =

(𝑇1 +𝑇2 ) 2 (𝑇4 +𝑇5 ) 4

(2) (3)

Where Qin is the heat load that is applied at the evaporator section and is calculated as Qin = Vin x Iin. Vin and Iin represent the input voltage and current respectively. The evaporator wall surface temperature (T1)” is the average of two temperatures measured at 25 mm from the evaporator side and “Evaporator surface temperature (T2)" is the average of the next two temperatures measured at 60 mm respectively. It can be noted that there are four thermocouples to measure the surface temperature at the evaporator section and the thermocouple positions are shown in Figure 2(b). Similarly, T3, T4, and T5 are calculated by finding the average of two temperatures measured at 85 mm, 125 mm and 175 mm respectively. The evaporator heat transfer coefficient (hev) is defined as the ratio of applied heat flux in the evaporator to the temperature gradient between the evaporator wall and the vapor. In this condition, the adiabatic wall is assumed as the vapor temperature for calculating the evaporator heat transfer coefficient by Eq. (4).

ℎev =

𝑄in 𝐴ev (𝑇e,w −𝑇e,v )

(4)

Where Te,v is the vapor temperature and Aev represents the heat transfer area that is exposed to vaporization. As the vapor temperature for the evaporator is not measured, the adiabatic wall temperature is assumed as vapor temperature for analysis. Similarly, the condenser heat transfer coefficient is defined as the ratio of heat rejected to the coolant water to the product of heat transfer area and temperature gradient between condenser vapor and wall which is given by, Eq (5). ℎco =

𝑄out 𝐴co (𝑇c,v −𝑇c,w )

(5)

Where Qout is the amount of heat rejected in Watts at the condenser section and Tc,v represents the condenser vapor temperature. Similar to that of the evaporator heat transfer coefficient, the adiabatic wall temperature is assumed as the vapor temperature for calculating the condenser heat transfer coefficient. Thermal efficiency (ηth) of thermosyphon is defined as the ratio of heat removal at the condenser section to the applied heat input, and it is calculated using Eq. (6) and the rate of heat rejected to the cooling water is determined by Eq. (7). 𝜂th =

𝑄out 𝑄in

x 100

𝑄out = 𝑚w 𝐶p (𝑇6 − 𝑇7 )

(6)

(7)

Where mw and Cp represents the mass flow rate and Specific heat capacity of cooling water respectively. The effective thermal conductivity of MPMC thermosyphon is calculated using Eq. (8).

𝐾eff =

𝐿eff 𝐴cs x 𝑅th

(8)

Where Acs is the cross-sectional area of the vapor flow area. Whereas, Leff indicates the effective length of thermosyphon calculated from the center of the evaporator and the condenser section.

2.4 Uncertainty Analysis. An experimental study is comprised of an uncertainty analysis as there are some unavoidable errors associated with the measuring instruments. These uncertainties are calculated for some derived parameters and are discussed below. As the heat input is determined by the product of voltage and current, there will be an uncertainty with heat load that corresponds to the measured voltage and current values. It is calculated using Eq. (9). 2

Δ𝑄in

2

Δ𝑉 Δ𝐼 = √( 𝑉 in ) + ( 𝐼 in )

𝑄in

in

(9)

in

The uncertainties for the thermal resistance of MPMC thermosyphon contain uncertainty by measuring the value of heat input and the temperature gradient between the evaporator and the condenser. It is calculated using Eq. (10). Δ𝑅th 𝑅th

2

2

Δ𝑄 Δ(Δ𝑇) = √( 𝑄 in ) + ( Δ𝑇 )

(10)

e,c

in

where Te,c is the actual difference in temperature between the evaporator and the condenser. The ratio of heat input or heat rejected to the product of temperature gradient between evaporator wall and vapor and with heat transfer area is defined as the heat transfer coefficient. Thus, the uncertainty involved can be calculated using the Eq. (11). Δℎe,c ℎe,c

2

2

Δ𝑞 Δ(Δ𝑇) = √( 𝑞 in ) + ( Δ𝑇 )

(11)

v,s

in

Where qin indicates the uncertainty in input heat flux and ΔTv,s represents the uncertainty in the temperature gradient between wall and vapor. Moreover, the uncertainty associated with the thermal efficiency is calculated by including the uncertainty of input heat load, mass flow rate (mw) of cooling water and the difference between the inlet and outlet temperature of the cooling water (ΔTcw), which is calculated using Eq. (12). Δ𝜂th 𝜂th

2

) 2

2

Δ𝑄 Δ𝑚 Δ(Δ𝑇 = √( 𝑄 in ) + ( 𝑚 w ) + ( Δ𝑇 cw ) in

w

cw

(12)

The maximum uncertainty associated with the heat load, thermal resistance, thermal efficiency, and heat transfer coefficients are 1.86%, 2.1%, 2.4% and 2.2% respectively. 3 Results and Discussion 3.1 Influence of filling ratio and inclination angle on the performance of MPMC thermosyphon The thermal performance of MPMC thermosyphon is greatly influenced by the FR and inclination angle. Figure 3 shows the operating temperature curve of different FR of 40%, 50% and 60% and inclination angle (optimum FR) of 45o, 60o and 90o. T1 and T2 represent the evaporator wall temperature at 25 mm and 60 mm respectively from one end of thermosyphon. From the results, it can be observed that the difference between T1 and T2 gets increased as the heat load increases from 10W to 50W. At higher heat load the difference between the evaporator wall temperature is noted as 2.69 °C, 2.67 °C and 2.45 °C for 40%, 50% and 60% FR respectively. But the OFR is found to be 50% as the average wall temperature value is noted as 58.89 °C which is lesser when compared to 40% and 60% FR. The rate of heat rejection from the inner wall of the evaporator to the working fluid determines the outer wall temperature. But heat rejection rate depends on the following factors such as FR, thermophysical properties of the working fluid, material’s thermal conductivity and vaporization area. Initially, the process was carried out with 40% FR, where the evaporator wall temperature ranges from 34.75 °C to 64.81 °C. When the FR was increased to 50%, the lowest wall temperature was observed ranging from 32.79 °C to 58.89 °C and it was 34.70 °C to 63.36 °C for 60%. From the results, it can be noted that the evaporator wall temperature is lower irrespective of all the heat loads when the FR is 50%. Thus, the FR of the working fluid influences more on heat transfer performance. For smaller FR (40%), MPMC thermosyphon reaches the equilibrium state quickly by which the evaporation rate occurs faster. In spite of the advantage, this condition performs well only for lower heat inputs as its heat transport capacity is limited due to less quantity of working fluid. Rather at higher heat loads for 40% FR, the density of the condensate film that descends along the wall decreases and this process will continue until the thermosyphon reaches its dry out limit. Beyond this limit, the performance of the thermosyphon will decrease, as the quantity of descended fluid will not be sufficient for removing the heat from the evaporator wall. Thus, the presence of a sufficient amount of working fluid is a necessary factor to make sure that the rewetting of the evaporator wall occurs.

3.1.1 Startup characteristics of MPMC thermosyphon For the application of electronic cooling using MPMC thermosyphon, it is an absolute fact to maintain the temperature of the electronic processor below 80 °C. However, the performance of a thermosyphon can be predicted by analyzing the response time with respect to the applied heat load. Optimization of FR and inclination angle for MPMC thermosyphon is done by carrying out experiments with different FR of 40%, 50%, and 60%. Experiments were done at heat loads of 10W, 20W, 30W, 40W and 50W. Figure 4(a) shows the startup characteristics of different FR and inclination angles for MPMC thermosyphon. It can be observed that for 40% FR the response time for the thermosyphon to reach the steady-state is quicker whereas, for the other two FR, the time taken to respond to the heat load is more. At low heat load and FR, the quantity of the working fluid is less thus taking lesser time to heat up. But for higher FR of 50% and 60% particularly at 10W, accumulation of working fluid in the evaporator section is more, resulting in slower response time for the temperature to rise. Figure 4(b) shows the startup characteristics of different inclination for the OFR of 50%. It can be observed that for the inclination angle of 45o and 60o the wall temperature reaches its highest value at a faster rate when compared to the vertical orientation (θ=90o). This is due to that, the quantity of working fluid exposed to the heat load varies as the thermosyphon is tilted to a certain angle. For vertical orientation, the quantity of fluid that is exposed is greater followed by 60 o and 45o. This results in increased time for the thermosyphon to reach its equilibrium state particularly for θ=90o. In addition, it can be noted that the wall temperature is higher which is noted as 33.85 °C and 34.24 °C for 60o and 45o respectively. Whereas the wall temperature for vertical orientation is noted to be 33.90 °C. But on further increase to 60%, the quantity of the working fluid inside the thermosyphon is increased causing more accumulation in the evaporator section. It is known that for higher FR, more time is required for the start-up process followed by low heat transfer from the evaporator section to the condenser section. This is because the area for vaporization is comparatively low for 60% than the smaller FR. Even though the change in the quantity of working fluid is very less for different FR, the vapor flow area should also be taken in to account for better heat transfer performance. As the adiabatic length of MPMC thermosyphon is fixed at 25 mm, the area for vapor flow will be reduced consequently by increasing the FR. Thus, the heat transfer performance of thermosyphon is less for 60% whereas the optimum FR was found to be 50%.

3.1.2 Thermal Resistance Analysis Thermal resistance analysis is one of the key factors for estimating the thermal performance of a thermosyphon. Figure 5(a) shows the change in resistance of MPMC thermosyphon for different FR and inclination angle. From the result, an usual trend of decreasing slope is observed for resistance when there is a linear increase in the heat load. This is due to an increase in heat transfer between the evaporator and condenser ends. The lowest thermal resistance value can be observed for 50% FR ranging from 0.907 °C/W to 0.391 °C/W. When the FR was decreased to 40%, a higher resistance value of 1.33 °C/W to 0.564 °C/W can be noted. Moreover, for 60% FR the value ranges between 1.11 °C/W and 0.52 °C/W. In addition, the thermal resistance value depends on the temperature gradient between the evaporator and the condenser section. For 40% FR, the quantity of vapor moving towards the condenser section will be comparably less. Thus, only less vapor gets condensed causing an increase in temperature difference between the evaporator and condenser section resulting in higher resistance value. For increased FR of 60%, the working fluid occupies some space of the adiabatic section causing greater vapor flow resistance of the fluid. As a result, the heat load that is given on the surface of the evaporator section gets accumulated and thus higher resistance value can be noted as 0.56 °C/W for 50W. Lowest thermal resistance of 0.391 °C/W is obtained at the maximum heat load for 50% FR and it shows a reduction of 30.1% when compared to the value obtained for the same heat load of 60% FR. Therefore, for higher FR there is a disturbance in the flow pattern and also weakening of heat transfer performance due to the presence of surplus fluid. But for the optimum FR, only the required amount of working fluid is available, which also provides space for the vapor to flow to the condenser even through the smaller channels. Even though the thermal resistance value showed a decreasing slope trend, there will be a particular point where the curve starts to move upwards resulting in poor performance of the thermosyphon. Particularly for smaller FR, the start-up process will be quicker promoting the natural circulation inside the thermosyphon. Whereas for higher heat loads, despite an increase in the evaporation ratio of the working fluid, the liquid layer of the downcomer from the condenser section shrinks gradually. This contraction results in a decrease in the radius of the meniscus that is coming down to the evaporator section and that is where the thermal resistance value rises up for maximum heat load. Inclination angle plays a vital role in influencing the heat transfer of

MPMC thermosyphon as shown in Figure 5(b). It can be noted that the performance of thermosyphon is not good after 40W heat load for the inclination angle of 45o. This is due to that, the fluid flow is slower through the mini channels for lower inclination angle indicating the reduction of heat transfer performance. In contrary to this, the flow of the working fluid from condenser to the evaporator occurs faster when the inclination angle is increased. That is the reason for the increase in thermal resistance value for the heat load of 50W at lower inclination. In addition, it will be the condition for the dry out phenomenon to occur. The working fluid flowing from the condenser experiences some disturbances due to frictional effects as the flow channel is minimum and also the influence of gravity is lesser when compared to higher inclination angles. But when the inclination angle is increased, even though frictional effects prevail, the influence of gravity dominates the flow resistance causing the working fluid to move faster towards the evaporator section for preventing the dry-out stage. The value of thermal resistance ranges from 1.125 °C/W to 0.907 °C/W for all inclination angles particularly for a lower heat load of 10W. This shows that there is a decrease in the resistance value of 19.3% for the inclination angle of 90o when compared to 45o. This variation keeps on decreasing till 40W and after this point, the difference started increasing as the resistance curve for 45o inclination moves up. Better performance can be noted only for the vertical orientation irrespective of all the heat loads. The resistance value for 90o inclination ranges between 0.907 °C/W and 0.391 °C/W. This proves the effect of the inclination angle on the performance of MPMC thermosyphon. 3.1.3 Convective heat transfer coefficient Figure 6(a) illustrates the effect of FR on evaporator heat transfer coefficients with different heat loads. As described above, 50% FR has the lowest thermal resistance value which indicates that there is a better heat transfer between the wall of the evaporator and the working fluid present in the thermosyphon. Therefore, the evaporator heat transfer coefficient reaches the highest value of 10.731 kW/(m2∙K) for 50% FR at the maximum heat load. It can be observed from the proposed design that, the thermosyphon has 10 multi-ports through which the fluid absorbs the latent heat and moves towards the condenser. The separators that isolate these channels, act as fins which links both the internal surface of the thermosyphon throughout its length. At the time of beginning the experiment, the entire working fluid will be stationary and present completely in the evaporator section. As the heat load is applied on the surface of the thermosyphon, the heat is transferred by

conduction on the walls and the conduction dominates convection (as there will be no moment in the working fluid). Gradually, the working fluid absorbs heat from the aluminum fins and gets vaporized. This less density vapor moves up towards the condenser section and gets condensed. This condensate descends along the surface and absorbs the heat from the fin through convection leading to higher evaporation rate and heat transfer coefficient. In addition, heat transfer enhancement is achieved by reducing the tube diameters and increasing the vapor quality of the liquid [18]. Moreover, flow resistance due to frictional effects will play a major role particularly in channels with smaller hydraulic diameter at lower heat loads. But then, as the evaporation rate of the working fluid increases at higher heat loads, the vapor tends to move at a faster rate towards the condenser section dominating the frictional effects. The viscosity of the working fluid also decreases when heat is applied which reduces the fluid friction along the flow. At low heat loads, the difference in heat transfer coefficient is comparably less and it is 0.857 kW/(m2∙K). Whereas, at higher heat loads, a gradual increase in the difference can be noted and the maximum difference in evaporator heat transfer coefficient is 3.09 kW/(m2∙K). Figure 6(c) represents a change in the condenser heat transfer coefficient by increasing the heat loads for three FR. The highest heat transfer coefficient value is observed for 50% FR and it is lower for 40% and 60%. The maximum heat transfer coefficient is noted to be 6.64 kW/(m2∙K) which is 4.42 kW/(m2∙K) greater than the value obtained for 40% FR. This is due to the increase in vapor pressure created in the evaporator section resulting in a higher vapor flow rate towards the condenser region. Thus, the more amount of vapor loses its latent heat and returns back to the evaporator. Lower heat transfer coefficient values indicate the excess and insufficient quantity of working fluid present inside the thermosyphon. Moreover, minimum evaporator wall thickness also results in lower thermal resistance, thus enhancing the heat transfer. In addition, the effect of the tilt angle on the evaporator heat transfer coefficient of MPMC thermosyphon for the OFR of 50% is been discussed. Enhancement in evaporator heat transfer coefficients can be observed for all inclination angles by increasing the heat load. This is due to the higher vaporization rate of the working fluid inside the evaporator section. By increasing the tilt angle of the thermosyphon, the flow of vapor becomes smoother thus resulting in faster flow from the evaporator to the condenser section. Moreover, the effect of surface tension also plays a major part to increase the heat transfer coefficient for vertical orientation (θ=90o) as shown in Figure 6(b) and 6(d). Higher the surface tension of the liquid, the higher will be the heat transfer performance, especially in rectangular

mini-channels. As a result, the condensate is pulled towards the corner of the channels leading to thinner liquid film on the flat sides. Thus, more heat is absorbed at the evaporator section that lowers the thermal resistance and results in a higher heat transfer coefficient on the perimeter of the channel. Also, surface tension plays a major role in preventing the condensate from accumulation at the bottom of smaller hydraulic diameter channels as reported by [17]. The range of the evaporator heat transfer coefficient lies between 4.705 kW/(m2∙K) to 10.731 kW/(m2∙K) for all the inclinations. The maximum value for all the heat loads is obtained for vertical orientation (90o) and it ranges from 6.668 kW/(m2∙K) to 10.731 kW/(m2∙K) which is 30.7% higher when compared to 45o inclination. The heat transfer coefficient value relies on the other following factors such as space for vapor flow, length of the condenser section to make sure sufficient cooling occurs, the thickness and surface property of the material, cooling specifications, and the insulation thickness. In this case, even at a higher heat load of 50W, the condenser length is capable of condensing all the vapors that approaches the evaporator. Moreover, subcooling dominates at the initial stages where only less quantity of vapor reaches the condenser. 3.1.4 Thermal conductivity and Thermal efficiency Figure 7(a) shows the change in the effective thermal conductivity of MPMC thermosyphon with respect to different heat loads for OFR of 50%. Effective thermal conductivity is a measure of how much heat is transferred quickly from the evaporator to the condenser section. The highest thermal conductivity value is obtained for the FR in which the lowest resistance is noted. 50% FR has the lowest thermal resistance and thus it has the highest effective thermal conductivity value of 2556.6 W/mK. It can be observed that the thermal conductivity value for 40% FR increases gradually but, at higher heat loads the value becomes steady. This indicates the insufficient amount of working fluid inside the thermosyphon and the performance will decrease if FR is further reduced to 30%. The thermal efficiency of MPMC thermosyphon with respect to different heat loads and FR is illustrated in Figure 7(c). It can be observed that there is an increase in thermal efficiency when heat load increases and maximum efficiency is obtained for OFR of 50%. As it is well known that thermal efficiency is a ratio of the amount of heat rejected at the condenser to the amount heat absorbed at the evaporator, maximum efficiency can be obtained only when a higher heat transfer rate occurs. A higher mass flow rate of vapor at the evaporator section increases the rejection rate at the condenser by which maximum thermal efficiency can be

obtained. The efficiency value ranges between 72.6% to 89.8% with heat loads from 10W to 50W for all the FR. The highest thermal efficiency is noted for 50% FR and the value ranges between 78% and 89.8%. This shows an increase in efficiency of 7.5% when compared to the 40% FR. Moreover, heat loss to the surrounding through the insulation should also be considered for thermal analysis as the thermal efficiency is a measure of actual heat that is transferred to the condenser. It can be understood that the thermal efficiency depends on the FR as the removal of heat relies on the vaporization of the working fluid and mass flow rate of vapor. As Keff depends on the tilt angle and the heat load, higher effective thermal conductivity values can be obtained only when an increase in heat transfer occurs that corresponds to the lower thermal resistance. Moreover, the higher Keff is obtained only for the vertical orientation (90o) for all the heat loads as shown in Figure 7(b). The effective thermal conductivity values range between 1036.4 W/mK and 2556.62 W/mK for the three FR and inclination angles. The maximum value of 2556.62 W/mK is observed for 50% FR with an inclination angle of θ=90o. In addition, a decrease in the curve can be noted for the inclination angle θ=45o, which indicates that the thermosyphon experiences the local dry out condition. Figure 7(d) describes the effect of heat load and tilt angle on the thermal efficiency of MPMC thermosyphon for OFR of 50%. It is well known that a thermal system giving 100% efficiency is not practically possible. This proves that all the heat that is given in the evaporator section is not fully transferred to the condenser section. But when compared to lower heat loads, the utilization of heat to turn the working fluid to vapor is more for higher heat loads, and thus more amount of heat is transferred causing the thermal efficiency value to increase. In the same way for different tilt angles, the flow of the working fluid is faster and it absorbs more heat in the vertical orientation (θ=90o) resulting in maximum thermal efficiency. The thermal efficiency value ranges between 74.2% to 89.8% for all the inclinations and heat loads. The highest thermal efficiency value is obtained for vertical orientation of thermosyphon and it ranges from 77.81% to 89.8% which is 11.13% increase when compared to 45o inclination for the maximum heat load. Even though the material is made of Aluminium, as the thickness of the outer wall is very small, there are chances for losing heat to the surrounding. From the results, it can be understood that insufficient quantity of working fluid and heat loss causes the thermal efficiency to decrease for the inclination angle of θ=45o for higher heat load. Whereas, for the vertical orientation of thermosyphon the heat loss is minimal due to optimum FR and due to the influence of gravity.

3.1.5 Wall temperature across the length of thermosyphon Figure 8(a) and 8(b) shows the variation in temperature along the length of thermosyphon. The lowest wall temperature is observed for the OFR of 50% and it ranges from 60.27 °C to 33.805 °C. For optimum filling, only a sufficient amount of working fluid will be present by which the evaporation rate occurs faster. The vapor due to its high pressure moves to the condenser and then it returns back for rewetting the inner walls of the evaporator section, thus reducing the wall temperature of the evaporator. It can be noted that the first two points of evaporator wall temperature are almost the same and there is a sudden drop in the temperature at the adiabatic section. This is because, the length of the adiabatic section is smaller when compared to the condenser section and as the thickness of the thermosyphon is very less, the cooling effect by conduction influences the wall temperature of the adiabatic section. Moreover, the temperature points at the adiabatic section look to be almost straight which indicates that there is only negligible loss of heat to the surrounding. When the inclination angle is increased, due to the high vapor generation rate, fluid that gets condensed comes back to the evaporator easily by the influence of gravity. But for lower inclination, the flow of descending fluid is not fully assisted by gravity. Rather, the flow of the working fluid is affected by two factors such as 1. At higher heat loads due to increased mass flow rate of vapor, the condensed fluid cannot reach the evaporator wall easily as the hydraulic diameter of the mini channel is too small for both the vapor and fluid to move in the opposite direction. 2. Due to the increase in heat load, the density of the working fluid that descends along the wall gets reduced. Therefore, higher wall temperatures can be observed and it can be observed that the minimum wall temperature of MPMC thermosyphon is observed at an inclination angle of 90o for 50% FR. The main factor that influences the evaporator wall temperature is the vaporization rate of the working fluid. The more the fluid turns to vapor, the more fluid will be condensed and returns back to reduce the wall temperature. In all heat loads, an increase in the rate of vapor generation must be always higher than the rate of heat transfer between the heat source and the outer wall of the evaporator. Thus, the increase in wall temperature indicates that the rate of heat transfer from the outer wall to the inner wall at the evaporator is less. Figure 9(a) shows the effect of FR on temperature difference between evaporator and condenser. It is observed that the minimum wall temperature difference is obtained for 50% FR

and the corresponding range is noted from 9.5 °C to 23.8 °C. Whereas for 40% and 60% FR, the difference in temperature ranges from 13.3 °C to 28.1 °C and 10.9 °C to 26.8 °C respectively. The reason for increased wall temperature difference at 40% FR is the insufficient amount of working fluid present inside the thermosyphon, resulting in a lesser vaporization rate. Thus, by increasing the heat load, the temperature difference keeps on increasing. But for 50% FR, due to the presence of a sufficient quantity of working fluid, the amount of vapor that travels towards the condenser section is more. A similar trend can be observed for the inclined case also as shown in Figure 9(b). When the thermosyphon is inclined to lower angles (θ=60o and θ=45o), the temperature difference is higher whereas for the vertical orientation it gives lesser value. For lower inclination angles, the quantity of working fluid that is in contact with the wall at the evaporator section is less. Therefore, on higher heat loads, the heat that is applied is not fully transferred to the working fluid rather it increases the wall temperature. Only less amount of working fluid will turn to vapor and move towards the condenser section for losing its latent heat. The maximum and minimum wall temperature difference is observed for the inclination angle of 45o and 90o and the values are noted as 29.6 °C and 23.8 °C respectively. 3.1.6 Vapor flow velocity and Index of Rate of Temperature rise Figure 10(a) shows the effect of FR and inclination angle on Vapor flow velocity. It can be observed that as the heat load increases the velocity of vapor also increases. Higher vapor velocity is observed for the optimum FR of 50% and it is noted to be 1.03 m/s. It is well known that the velocity of vapor directly depends on the mass flow rate of vapor. Moreover, for an increased mass flow rate, a sufficient quantity of working fluid inside the thermosyphon is essential. For lower FR (40%) due to less quantity of working fluid and evaporation rate, the mass flow rate of vapor is reduced and the value is noted as 0.85 m/s. Moreover, for lower inclination, the velocity is less due to more frictional effects which restricts the free flow of vapor towards the condenser section. Even though the presence of mini channels inside the thermosyphon enhances the evaporation rate, for lower inclination angle the vapor will experience the flow disturbance in all the channels thus resulting in lower flow velocity. Vapor velocity (𝑉vap ) is calculated using the following equations. 𝑚rate =

𝑄in (ℎg −ℎf )

(13)

𝑉vap =

𝑚rate 𝜌v x 𝐴c,s

(14)

Where, 𝑚rate , ℎg , ℎf , 𝜌v , 𝐴c,s are mass flow rate of vapor, enthalpy of vapor, enthalpy of liquid, the density of vapor and cross-sectional area of thermosyphon respectively. The thermophysical properties of the working fluid are taken for the wall temperature measured for a particular heat load. Moreover, the effect of FR and inclination angle on the thermal performance of MPMC thermosyphon can be compared with the Index of rate of temperature rise (α) as shown in Figure 10(b). The index of rate of temperature rise can be related directly with the velocity of vapor moving towards the condenser section. The higher the velocity of vapor the greater will be the amount of vapor that gets condensed and move towards the evaporator section. Thus, rewetting the wall of the evaporator occurs resulting in decreased wall temperature. Moreover, the index of rate of increase in temperature also decreases. The index of rate of temperature rise can be calculated by the ratio of increase in evaporator temperature(ΔTe) to the increase in the heat load (ΔQin) and is calculated by Eq. (15), 𝛼=

(𝑇e,max − 𝑇e,min ) (𝑄in,max − 𝑄in,min )

(15)

Therefore, for the optimum FR of 50%, it can be described that the temperature control ability is higher when compared to the other FR and inclination angles. It can be stated that the smaller the index of rate of temperature rise, the stronger the temperature control ability of MPMC thermosyphon. 3.1.7 Maximum heat transfer capacity Even though the thermosyphon is considered to be efficient heat transfer devices, it is necessary to study the limitations on the operation of thermosyphon. Moreover, these limitations must be studied for every design of a thermosyphon and the working fluid. The minimum of the limitations indicates the maximum heat transfer capacity of a thermosyphon. Figure 11(a) shows the maximum heat transfer capacity of MPMC thermosyphon by varying the FR and inclination angle. Among these limits, the flooding limit is the most important limitation, particularly for the smaller flow channels. Flooding limit occurs when the vapor which is moving upwards towards the condenser section is higher enough to push the descending fluid due to shear stress at fluid-

vapor interface. Thus, for higher heat loads due to mini channels in MPMC thermosyphon and increased mass flow rate of vapor, there is a greater possibility for the vapor to push the condensed fluid towards the condenser section. Moreover, the flooding limit depends on Bond number which is the ratio of gravitational force to the surface tension force and the effect of FR and inclination angle on Bond number by increasing the heat load is shown in Figure 11(b). Flooding limit is calculated from the Eqs. (16)–(18) [29]. 𝑄flood = 𝐾ℎfg 𝐴c,s [𝑔𝜎(𝜌liq −𝜌vap )] 𝜌

𝐾 = (𝜌 l )

0.14

v

0.25

(𝜌liq −0.25 + 𝜌vap −0.25 )

𝑡𝑎𝑛ℎ2 (𝐵𝑜0.25 )

𝐵𝑜 = 𝐷h [

𝑔(𝜌l −𝜌v ) 0.5 𝜎

]

−2

(16) (17)

(18)

Where K is the Kutateladze number and Bo is the Bond number. The boiling limit and sonic limit are calculated from the Eqs. (19) & (20) [30]. 𝑄boil = 0.12(2л𝑟h )ℎfg 𝜌v 0.5 [𝜎𝑔(𝜌l − 𝜌v )]0.25 𝑄sonic = 𝜌v ℎfg 𝐴v (

𝛾𝑅v 𝑇v 0.5

)

2(𝛾+1)

(19) (20)

From the results, it can be observed that the heat transfer study for MPMC thermosyphon is well below the minimum limit obtained. The above discussion indicates that the FR and the tilt angle are the two influencing factors which affect the thermal performance of MPMC thermosyphon. Moreover, the optimum FR for this proposed design is 50% of evaporator volume and the highest heat transfer performance is noted for the inclination of 90o.

4 Conclusions The heat transfer performance characteristics of a novel ultra-thin multiport minichannel thermosyphon for cooling miniaturized power electronic products is experimentally studied. The effect of heat load, filling ratio and tilt angles on thermal resistance, convective heat transfer coefficients of evaporator and condenser, vapour velocity, flooding, sonic, boiling limits and efficiency are investigated. Based on the obtained results, the following conclusions are drawn.

1. Optimum filling ratio (OFR) of 50% was obtained at vertical orientation. Reduction in evaporator wall temperature and thermal resistance of 9.31% and 22.2% are respectively observed at OFR. 2. Enhancement in evaporator heat transfer coefficient and thermal efficiency of 30.7% and 89.8% are noted. 3. Increase in the vapour velocity of 12% is observed at the optimum filling ratio and a decrease of 19.2% in the wall temperature difference between evaporator and condenser is also noted at an average heat flux of 21.4kW/m2. 4. Multiports present in minichannel acts as internal fins and increases the surface area and evaporation rate. 5. Further multiports increases the surface tension of condensate at right angles to the flow direction along with the effects of gravity enhancing the rate of condensation. Thus, the obtained experimental results show that the ultra-thin multi-port mini channel (MPMC) thermosyphon will be useful in the cooling of high heat flux electronic products. Acknowledgment The authors gratefully acknowledge the financial support provided by the funding agency, the Department of Science and Technology (DST), Science and Engineering Research Board (SERB), (SB/FTP/ETA-362/2012), New Delhi, India. The authors would like to thank Mr. Jayaseelan R, Mr. Sheno Jerbin and Mr. David of Karunya Institute of Technology and Sciences for helping in the fabrication of the experimental test facility.

Nomenclature Cp

= Specific heat capacity (J/kgK)

h

= heat transfer coefficient (W/(m2∙K))

m

= mass flow rate of cooling water (kg/s)

qin

= input heat flux (W/m2)

A

= Heat transfer area (m2)

Iin

= Current (A)

Vin

= Voltage (V)

L

= length measured from evaporator to condenser (m)

T

= temperature (°C)

g

= gravity (m/s2)

hfg

= Latent heat (J/kg)

Qin

= Input heat load (W)

Qout

= heat rejected at condenser (W)

Rth

= thermal resistance (°C/W)

rh

= hydraulic radius

LPH

= liters per hour

FR

= Filling Ratio

TPCT = Two Phase Closed Thermosyphon MPMC

= multi-port minichannel

MVCTHP

= Magnetically Variable Conductance Thermosyphon Heat Pipe

Greek Symbols ηth

= thermal efficiency

Δ

= difference

θ

= Inclination angle

Subscripts v

= Vapor

co

= Condenser

w

= water

e,w

= evaporator wall

c,w

= condenser wall

cs

= cross-sectional

v,s

= between vapor and surface

e,c

= between evaporator and condenser

boil

= boiling

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List of Figures

Figure 1.

Ultra-thin multi-port minichannel thermosyphon

Figure 2.

(a) Experimental setup of ultra-thin multi-port minichannel thermosyphon with port dimensions (b) Position of thermocouples

Figure 3.

Operating Characteristics of MPMC thermosyphon

Figure 4.

Time vs Temperature (a) Different filling ratios (b) Different inclination angle

Figure 5.

Heat load vs Thermal resistance (a) Different filling ratios (b) inclination angles

Figure 6.

Heat load vs Heat transfer coefficients

Figure 7.

Heat load vs Thermal conductivity and Thermal efficiency

Figure 8.

Length vs Temperature

Figure 9.

Heat load vs Temperature difference (Te – Tc)

Figure 10.

Heat load vs (a) Velocity of vapor (b) Index of rate of Temperature rise

Figure 11.

(a)Variation of Qmax for different filling ratios and inclination angle (b) Heat load vs Bond number

Filling tube

Outlet valve Condenser Thermocouple Adiabatic section

Heating coil with Mica sheet Evaporator End closure

Figure 1. Ultra-thin multi-port minichannel thermosyphon

Thermosyphon Rotameter

Cooling water outlet

25 mm

Condenser surface (T5)

Condenser section

Data Acquisition system

Cooling water inlet

1 Evaporator section

Storage tank withChilling chilling unit unit

Adiabatic section

50 mm

40 mm

Adiabatic surface (T3)

Heat load

25 mm

Thermocouple positions

1

Condenser surface (T4)

Dimmerstat

Evaporator surface (T2)

MPMC thermosyphon

35 mm

Multiports dimensions in mm

Evaporator surface (T1) 25 mm

(a)

(b)

Figure 2. (a) Experimental setup of ultra-thin multi-port minichannel thermosyphon with port dimension (b) Position of thermocouples

60

30W

50

Evaporator surface (T₁) Adiabatic surface (T₃) Condenser surface (T₅)

70

40W

FR=40% IA=90⁰

20W

40 10W

30

80

Evaporator surface (T₂) Condenser surface (T₄) Coolant outlet (T₆)

60

FR=50% IA=90⁰

40W

50

20

30W 20W

40 10W

40

60

80

100

20

40

Time (minutes) 80

60

FR=50% IA=60⁰

Evaporator surface (T₂) Condenser surface (T₄) Coolant outlet (T₆)

30W

50 20W

40

10W

100

0

20

40

Evaporator surface (T₁) Adiabatic surface (T₃) Condenser surface (T₅)

70

30W

20W

10W

30

60

Time (minutes)

50W

50 40

80

80

40W

60

FR=60% IA=90⁰

Time (minutes) Evaporator surface (T₁) Adiabatic surface (T₃) Condenser surface (T₅)

70

40W

60

20 0

Temperature ( C)

20

Evaporator surface (T₂) Condenser surface (T₄) 50W Coolant outlet (T₆)

30

20 0

Evaporator surface (T₁) Adiabatic surface (T₃) Condenser surface (T₅)

70

50W

30

Temperature ( C)

Temperature ( C)

70

80

Evaporator surface (T₂) Condenser surface (T₄) Coolant outlet (T₆) 50W

Temperature ( C)

Evaporator surface (T₁) Adiabatic surface (T₃) Condenser surface (T₅)

Temperature ( C)

80

Evaporator surface (T₂) Condenser surface (T₄) Coolant outlet (T₆)

50W

40W

60

FR=50% IA=45⁰

50

30W

20W

40 10W

30

20 0

20

40

60

Time (minutes)

80

100

20 0

20

40

60

Time (minutes)

Figure. 3 Operating Characteristics of MPMC thermosyphon

80

100

80

100

36

34

34

32

32

Temperature ( C)

Temperature ( C)

36

30 28 26

FR=40%

30 28 26 FR=50%,θ=45

24

FR=50%

24

22

FR=60%

22

FR=50%,θ=60 FR=50%,θ=90 20

20

1

2

4

8

16

Time (s) (a)

32

64

128

1

2

4

8

16

32

Time (s) (b)

Figure. 4 Time vs Temperature (a) Different filling ratios (b) Different inclination angle

64

128

1

1.35

FR = 50%

1.05

FR=50%,θ=45°

0.9

FR = 40%

Thermal resistance ( C/W)

Thermal resistance ( C/W)

1.2

FR = 60% 0.9 0.75 0.6 0.45

FR=50%,θ=60° FR=50%,θ=90°

0.8 0.7 0.6 0.5 0.4

0.3

0.3 0

10

20

30

40

50

60

0

10

20

30

40

Heat load (W)

Heat load (W)

(a)

(b)

Figure. 5 Heat load vs Thermal resistance (a) Different filling ratios (b) Inclination angles

50

60

12 FR = 40%

11

Evaporator heat transfer coefficient kW/(m2 ∙K)

Evaporator heat transfer coefficient kW/(m2 ∙K)

12

FR = 50% FR = 60%

10 9 8 7

6

FR=50%,θ=45°

11

FR=50%,θ=60° FR=50%,θ=90°

10 9 8 7

6 5 4

5 0

10

20

30

40

50

0

60

10

FR = 40% FR = 50%

FR = 60%

4

3 2

1 0

0

10

20

30

40

50

60

Condenser heat transfer coefficient kW/(m2 ∙K)

Condenser heat transfer coefficient

kW/(m2 ∙K)

7

5

30

40

50

60

50

60

Heat load (W) (b)

Heat load (W) (a)

6

20

8 FR=50%,θ=45° 7 FR=50%,θ=60°

6 FR=50%,θ=90° 5

4 3

2 1

0 0

10

Heat load (W) (c)

Figure. 6 Heat load vs Heat transfer coefficients

20

30

40

Heat load (W) (d)

3000

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

3000 FR = 40%

2500

FR = 50% FR = 60%

2000

1500

1000

FR=50%,θ=45° 2500

FR=50%,θ=60° FR=50%,θ=90°

2000

1500

1000

500

500 0

10

20

30

40

50

60

0

10

20

100

100 FR = 60%

50

60

FR=50%,θ=45°

FR=50%,θ=60°

FR=50%,θ=90°

90

80

Thermal efficiency (%)

Thermal efficiency (%)

90

FR = 50%

40

Heat load (W) (b)

Heat load (W) (a)

FR = 40%

30

70

60 50 40

30 20 10

80

70 60 50

40 30 20

10

0

0 10

20

30

Heat load (W) (c)

40

50

10

20

30

Heat load (W) (d)

Figure. 7 Heat load vs Thermal conductivity and Thermal efficiency

40

50

75

70 65

FR=50%,θ=45

65

FR=50%,θ=60

FR = 40%

60

FR = 50%

Temperature ( C)

Temperature ( C)

70

FR = 60%

55 50 45 40

FR=50%,θ=90

60 55 50 45 40

35

35

30

30 0

25

50

75

100

125

150

175

200

0

25

Length (mm)

50

75

100

125

Length (mm)

(a)

(b) Figure. 8 Length vs Temperature

150

175

200

30

35 FR=50%,θ=45

FR = 40% FR = 50%

25

FR=50%,θ=60

30

FR=50%,θ=90 25

Te - Tc ( C)

Te - Tc ( C)

FR = 60% 20

15

20

15 10 10 5 0

10

20

30

40

Heat load (W)

50

60

5 0

10

20

30

40

Heat load (W)

(a)

Fig. 9 Heat load vs Temperature difference (Te – Tc)

(b)

50

60

1.1

Index of rate of Temperature rise (α)

3.5

Velocity of vapor (m/s)

1 0.9 0.8 FR=40%

0.7

FR=50% FR=60%

0.6

FR=50%,θ=45° 0.5

FR=50%,θ=60°

3

2.5

2 FR=40%

1.5

FR=50% FR=60% 1

FR=50%,θ=45° FR=50%,θ=60°

0.4

0.5 0

10

20

30

Heat load (W) (a)

40

50

60

0

10

20

30

40

Heat load (W (b)

Figure. 10 Heat load vs (a) Velocity of vapor (b) Index of rate of Temperature rise

50

60

180 flooding limit

Boiling limit

0.58

Sonic limit

160

140

FR=40%

0.56

FR=50% FR=60%

Bond number

120

Qmax (W)

0.57

100 80

60

0.55

FR=50%,θ=45°

0.54

FR=50%,θ=60°

0.53 0.52 0.51 0.5

40 0.49 20

0.48

0

0.47 0

10

20

30

40

50

Heat load (W) (b) Filling ratio (%) (a)

Figure. 11 (a) Variation of Qmax for different filling ratios and inclination angle (b) Heat load vs Bond number

60

List of Tables

Table 1.

Details of ultra-thin multi-port minichannel thermosyphon

Table 1. Details of ultra-thin multi-port minichannel thermosyphon Specification

Dimension/Material

Working fluid

Acetone

Filling Ratios

40%, 50% and 60%

Inclination tested

45o, 60o and 90o

Heat load

10W, 20W, 30W, 40W and 50W

Minichannel Material

Aluminium

Total length of Minichannel

200 mm

Width of Minichannel

20 mm

Width of one channel

1.45 mm

Height of one channel

1 mm

Base thickness

0.5 mm

Fin width (Wfin)

0.5 mm

Number of channels

10

Hydraulic diameter (Dh)

1.18 mm

Condenser Type

Cylindrical type

Material

Acrylic

Outside dimensions (length x OD)

100 mm x 35 mm

Wall thickness

3 mm

Coolant

Water

Coolant temperature

20°C

Coolant flow rate

10 LPH

Research highlights 

Multi-ports act as internal fins & increases surface area and evaporation rate



22.2% decrease in thermal resistance is observed at optimum filling ratio (OFR)



9.31% decrease in wall temperature is noted in degree Celsius scale at OFR



30.7% increase in evaporator heat transfer coefficient & 90% efficiency at OFR